Inert dental glass

ABSTRACT

The invention relates to the use of ions of weakly basic oxides as linking ions for polyacids in cements, preferably polyelectrolyte cements. Suitable ions comprise elements of the scandium series, for example, Sc 3+ , Y 3+ , La 3+ , Ce 4+  and all subsequent tri- and tetra-valent lanthanides and the ions Mg 2+ , Zn 2+ , Ga 2+ , In 2+ . The application of said ions permits a regulation of the cement reaction without surface treatment of the glass powder.

[0001] The invention relates to the use of unreactive glasses in dentalmaterials, in particular dental cements, preferably polyelectrolytecements, which can be used without pretreatment of the glass powdersurface.

[0002] In the dental sector, glasses are used in particular for fillingmaterials and for the fixing cements and composites for crowns, bridgesand inlays.

[0003] Reactive glasses, i.e. glasses which participate in a chemicalreaction, are used in what are known as polyelectrolyte cements, inparticular glass-ionomer cements.

[0004] Polyelectrolyte cements of this type generally comprise threeconstituents, namely a polyacid, in particular a substance whichcontains carbonic acid, preferably in liquid form, a glass powder andwater. If the three components are combined and mixed with one another,a reaction occurs, so as to form a solid body which hardens over thecourse of time (cement reaction).

[0005] Various raw materials are used for the production of glasseswhich are used in particular in glass-ionomer cements. These rawmaterials are firstly oxides, such as SiO₂, Al₂O₃, CaO, fluorides, suchas CaF₂, SrF₂, cryolite, hydroxides, such as Al(OH)₃, phosphates, suchas AlPO₄, P₂O₅ or calcium phosphates. However, it is also possible touse silicates, such as mullite, or carbonates, such as Na₂CO₃, CaCO₃ orother natural mineral raw materials. In principle, it is also possiblefor all the raw materials to be used in a form which contains water ofcrystallization.

[0006] In dental glasses, a considerable proportion of the oxygen isoften replaced by fluorine. This is indicated by adding the elementsymbol F for fluorine to the description of the glass system.

[0007] Accordingly, glasses for glass-ionomer cements can usually beassigned to one of the following systems, in which P₂O₅ and Na₂O in somecases are only present in small amounts or are not present at all:

[0008] SiO₂—Al₂O₃—CaO—(P₂O₅)—(Na₂O)—F

[0009] SiO₂—Al₂O₃—SrO—(P₂O₅)—(Na₂O)—F

[0010] SiO₂—Al₂O₃—SrO—La₂O₃—(P₂O₅)—(Na₂O)—F

[0011] SiO₂—Al₂O₃—CaO—La₂O₃—(P₂O₅)—(Na₂O)—F

[0012] The glasses which are used in dental cements are generallyfluoroaluminosilicate glasses. The solubility of the glass in acid is aprecondition for it to be used as a constituent of a polyelectrolytecement. An acid-soluble glass structure is formed if silicon ispartially replaced by aluminum. However, silicon can only be replaced byaluminum if basic oxides are present, in order to create chargeequalization for the trivalent aluminum ion at positions of thetetravalent silicon ion.

[0013] When the polyacids and water are added, the glass structure isbroken up, and in particular the ions with network-modifying propertiesare at least partially released as what are known as crosslinker ions.

[0014] The crosslinking manifests itself in hardening of the cementwhich increases over the course of time. All at least divalent basicions, but also Al³⁺, are able to form polymeric structures of this type.

[0015] A distinction is usually drawn between the working time—the timeduring which the dentist is still able to work the still pasty cementmaterial—and the hardening time—the time beyond which reworking ispossible using rotating dental instruments.

[0016] It has been found that conventional glasses, which contain, forexample, Ca²⁺ and Al³⁺ +s crosslinker ions, in untreated form are tooreactive and, on account of an excessive solubility, set too quicklywith the polyacid, and consequently the dental cement which forms cannotreasonably be worked.

[0017] Although it is possible to slow the dissolution process byreducing the calcium content in the glass, it has been found that if thelevel of basic oxides, such as CaO or SrO, which can dissolve, is toolow, the strength properties of the cement deteriorate as a result ofinsufficient availability of the ions. This means that the dentist hasonly a very short working time available to mix the filling material andapply it. At the same time, he has to accept the drawback of having towait a very long time before he can start reworking the cement. Thisruns contrary to the demands which a dentist will impose on a dentalcement.

[0018] The dentist usually requires a working time of from 1 to 4 minand a hardening time of from 5 to 8 min. The hardening time is usuallydetermined according to ISO 9917 (First Edition) Part 7.3. The workingtime and the hardening time can be determined using a viscometer, asdescribed in EP 0 023 013 A.

[0019] To achieve the desired working properties of the cement, i.e. tohave sufficient working time and the shortest possible time to completehardening, it is customary for the glass powders, after the millingprocess, to be subjected to surface treatment, as described, forexample, in Clinical Materials 12, 113-115 (1993) or DE 29 29 121 A (EP0 230 113 A). In this case, the glasses which react too quickly, onaccount of their composition, are adjusted to the desired level ofreaction rate by reducing the levels of reactive ions at their surface.

[0020] EP 0 023 013 A describes the use of a calcium aluminumfluorosilicate glass powder for glass-ionomer cements to which furtheroxides may be added if they do not adversely affect the properties ofthe glass. According to the description, the surface of the glass has tobe deactivated in order to obtain a glass which can be used for a dentalcement. The deactivation represents a procedure whereby the reactionrate of a glass powder with an acid is delayed by a surface treatment,and in this way the desired working times of the cement are produced.

[0021] The deactivation of the surface can also be effected by means ofother, relatively complex surface treatments, such as coating thesurface, for example with a polymer.

[0022] In EP 0 023 013 A, this is achieved by means of a chemicaltreatment of the powder surface. The result is a cement with favorableworking times combined, at the same time, with unchanged favorablemechanical characteristic data of the material.

[0023] However, this surface treatment of the glasses represents acomplex process step.

[0024] Moreover, during the washing or conditioning processes, powderagglomeration may occur, having an adverse effect on the cementproperties.

[0025] DE 38 06 448 A has disclosed a glass for a bone cement whichcomprises the elements Si, Al, Ca, Sr, F, Na and P and can be madevisible to X-rays by the addition of La₂O₃. It is emphasized that thequantity of additives must not adversely affect the properties.

[0026] The glass powder described in DE 38 04 469 A is substantiallyfree of alkali metal ions and alkaline-earth metal ions, with theexception of strontium which should be used in an amount from 15 to 40%by weight.

[0027] DE 20 65 824 B2 has described a fluoroaluminosilicate glasspowder for self-hardening medical cements.

[0028] One object of the present invention is to provide a glass for adental cement, in particular a reactive glass for a polyelectrolytecement, which is simple to produce.

[0029] A further object can be considered to lie in that of directlyusing the glass immediately after the milling process without having toapply complex processes such as surface treatment, acid washing, coatingand/or conditioning. The reactivity and therefore the working time andhardening time are then dependent only on the glass composition and thegrain size distribution and are easy to produce reproducibly.

[0030] This object is achieved by a dental material containing a glassas described in the claims, and by the use of certain ions ascrosslinker ions in a glass.

[0031] The invention also relates to hardenable materials, in particularcements, in particular polyelectrolyte cements, which contain theseglasses.

[0032] In the context of the present invention, the term crosslinking isto be understood as meaning a reaction in which polyacids and at leastdivalent ions interact with one another in a chelate-forming reaction,preferably an acid-base-type reaction, leading to the formation of apolymeric network.

[0033] In the context of the present invention, the filling and fixingmaterials mentioned are to be understood as meaning substantiallycements, and in particular polyelectrolyte cements. Accordingly, theglass described is preferably a reactive constituent rather than aconventional filler, unlike the glasses used in the composite sector,which are pure fillers and do not take part in a reaction.

[0034] Glasses for cements generally contain strongly basic ions, suchas Li⁺, Na⁺, K⁺, Ca²⁺, Sr²⁺, Ba²⁺, Zn²⁺. It has now been found that bycompletely or partially replacing the strongly basic ions with weaklybasic ions, such as Sc³⁺, Y³⁺, La³⁺, such as Ce^(3+/4+), or otherdivalent, trivalent or tetravalent ions from the lanthanide series,and/or Ga²⁺ or In²⁺, glasses which set significantly more slowly withpolyacids are obtained.

[0035] Surprisingly, such glasses can be used to produce dental cementswhich, substantially without a conventional surface treatment of theglass powders, have setting characteristics which are desired by thedentist. Furthermore, it has been found that the setting times can beadjusted within a wide range by means of the glass composition.

[0036] In this context, the invention has the following advantages:

[0037] As a result of the strongly basic ions, such as Ca⁺, Sr²⁺, beingreplaced by the weakly basic ions Sc³+, Y³⁺, La³⁺, Ce^(4+/3+) and otherdivalent, trivalent and tetravalent ions from the lanthanide series inglasses which are used in dental cements, it is possible to achieve acontrolled setting reaction of the dental cement, in particular of aglass-ionomer cement, without the glass having to be surface-treated,for example by acid washing and/or conditioning, before being used inthe cement. In addition to simplified production, the advantage alsoresides in the improved reproducibility of the working and hardeningtimes. Although these times are not adjusted by a surface treatment, thedesired setting profile, namely a relatively rapid transition from astate in which the cement can still be worked to a state in which thehardening begins and useful working is no longer possible, issurprisingly achieved.

[0038] Amazingly, it has been discovered that dental materials or cementin which the abovementioned glasses are used have mechanical propertieswhich are identical or even slightly improved compared to cements inwhich glasses whose reactivity has been reduced by acid washing areused.

[0039] Furthermore, it has been found that cements according to theinvention are hydrolytically stable with respect to water.

[0040] These properties were found in particular in glasses which, inaddition to Al and Si, contain only Y and/or La or contain onlyrelatively small amounts of relatively strongly basic-reacting ions,such as Ca²⁺ or Sr²⁺, Ba⁺, Li⁺, Na⁺, K⁺.

[0041] Furthermore, some of the oxygen can be replaced by fluorine,which on the one hand improves the meltability of the glass and on theother hand improves the setting properties of the cement and makes itpossible to release fluoride ions for secondary caries prophylaxis.

[0042] Therefore, the previously known cement systems are expanded bythe addition of the following systems.

[0043] SiO₂—Al₂O₃—(SrO)—Ln_(x)O_(y)—P₂O₅—(Na₂O)—F

[0044] SiO₂—Al₂O₃—(CaO)—Ln_(x)O_(y)—P₂O₅—(Na₂O)—F

[0045] Ln_(x)O_(y) stands for an oxide of the elements Sc, Y, La to Lu.x and y may adopt values of 1, 2 or 3 in this formula. The oxides whichare inside parenthesis are used in only small amounts or are not used atall, since they would greatly accelerate the reaction. For example, DE20 65 824 A describes a glass belonging to the system

[0046] SiO₂—Al₂O₃—La₂O₃—P₂O₅—Na₂O—F

[0047] having an Na₂O content of approx. 12% by weight.

[0048] Tests have shown that with this glass powder the setting ratewith polyacids can only be brought into a manageable range afterconditioning for several hours at 400° C. (cf. Comparative Example 4).This is presumably attributable to the high level of a strongly basicoxide, in this case Na₂O. A further drawback of a high Na₂O content isthe increased water solubility of the resulting cement.

[0049] Moreover, it has been found that the glasses describedsubstantially do not have any phase-separation or crystallization effectwithin a wide range of compositions.

[0050] It is to be expected that the reproducibility of the setting rateof the cement containing the glasses will improve with clear glassescompared to segregated, i.e. opaque glasses, since their phasecomposition is not dependent on the cooling rate.

[0051] In addition to the standard components SiO₂, Al₂O₃, P₂O₅, andNa₂O, the glasses preferably mainly contain weakly basic and/oramphoteric-reacting ions, which act as crosslinker ions during thecement reaction.

[0052] Weakly basic trivalent and tetravalent ions are preferred, andthe ions Sc³⁺, Y³⁺, La³⁺, Ce^(4+/3+) and all the following trivalent andtetravalent ions from the lanthanide series are particularly preferred.

[0053] Current teaching is that Al³⁺ also belongs to the weakly basic oramphoteric-reacting ions. However, this ion adopts a special positionamong the glasses. Aluminum is primarily responsible for the solubilityof the glass structure in acid and has only a secondary function as acrosslinker ion. In the glasses which are suitable for dental cements,aluminum, unlike the abovementioned trivalent and tetravalent ions,which function as network modifiers, acts as a network former.

[0054] The glasses used generally have a BET surface area of from 1 to15 m²/g, preferably 2 to 8 m²/g.

[0055] Furthermore, the glasses have a mean grain size (d₅₀) of from0.01 to 20 μm, preferably 1 to 5 μm.

[0056] It is preferable for 0 to 25% by weight of the oxygen in theglass used to be replaced by fluorine, particularly preferably from 8 to18% by weight.

[0057] The pK_(B) value is usually used to define the term basicity. ApK_(B) of one can be taken as the limit between weakly and stronglybasic. For example, the pK_(B) of Mg(OH)₂ is given as one in R. C.Weast: CRC Handbook of Chemistry and Physics, while Ca(OH)₂ isclassified as strongly basic, without being assigned a numerical value.

[0058] In the context of the present invention, oxides or hydroxideswhich only dissociate to a relatively minor extent in aqueous solutionsare considered to be weakly basic.

[0059] The following statements can be made in connection with thebasicity:

[0060] The basicity increases from Sc through Y to La. La is to beclassified as weakly basic compared to Sr, Ba or Na and K. At the sametime, the basicity decreases again from La to Lu, and consequently thebasicity of lutetium is approximately comparable to that of yttrium(lanthanide contraction).

[0061] Therefore, all oxides and hydroxides of the Sc series can beconsidered weakly basic in the context of present invention.

[0062] The elements from the 1st main group from Li through Cs and theelements from the second main group from Mg through Ba cannot beclassified as weakly basic-reacting in the context of the presentinvention.

[0063] As has already been stated, it is assumed that, in addition tothe base strength, the higher field strength of these ions also plays acertain role. This means that the ions described are anchored morestrongly in the glass structure and are therefore dissolved out of itmore slowly.

[0064] In the context of the present invention, the term polyacid isunderstood to mean a polyelectrolyte which includes a polymer withionically dissociable groups, which may be substituents in the polymerchain and the number of which is so great that the polymers, at least intheir (partially) dissociated form, are at least partiallywater-soluble. Substituents such as for example —COOH, —OH, —PO(OH)₂,—OPO(OH)₂, —SO₂(OH) are particularly suitable for this purpose. Organicpolyacids (DE 20 61 513 A), such as polymers and copolymers of acrylicacid, methacrylic acid (EP 0 024 056 A), itaconic acid, maleic acid,citraconic acid, vinylphosphonic acid (EP 0 340 016 A; GB 22 91 060 A)are particularly preferred. In addition, if a plurality ofpolyelectrolytes are present, water-insoluble polyelectrolytes may alsobe present in the polyelectrolyte cement. The only condition is that atleast one of the polyelectroytes be at least partially water-soluble.

[0065] The polyelectrolytes should be able to react with the glasscomponent as part of a chelate-forming reaction and/or an acid-basereaction/neutralization reaction.

[0066] The polyelectrolyte cement contains the at least partiallywater-soluble polyelectrolyte, which can be converted into the solidstate, preferably in an amount from 0.5 to 30% by weight, particularlypreferably 2 to 25% by weight and very particularly preferably 5 to 20%by weight.

[0067] In the case of polyelectrolyte cements, the addition of chelatingagents in order to establish a suitable setting characteristic isparticularly important (DE 23 19 715 A). There are numerous compoundswhich are suitable for this purpose, in particular those which containhydroxyl or carboxyl groups, or both, which form the chelating agents.Particularly good results have been achieved with tartaric acid orcitric acid, in particular in an amount of 5% by weight. Adding thesubstance in the form of a metal chelate also has the desired effect.

[0068] The polyelectrolyte cements contain from 0 to 10, preferably 0 to5% by weight of a compound of this type, preferably tartaric acid.

[0069] Furthermore, the polyelectrolyte cement may include auxiliaries,such as dyes, pigments, X-ray contrasting agents, flow improvers,thixotropic agents, polymer thickeners or stabilizers.

[0070] Examples of standard fillers for dental materials are glass andquartz powder, plastic powder, pyrogenic highly dispersed silicas andmixtures of these components.

[0071] These other additives are usually present in the polyelectroytecements according to the invention in amounts of from 0 to 60% byweight.

[0072] The abovementioned fillers may also be rendered hydrophobic bymeans of a surface treatment with organosilanes or organosiloxanes or byetherification of hydroxyl groups to form alkoxy groups.

[0073] In principle, the glass composition which has been described isalso suitable for use in monomer-modified cements.

[0074] It has proven favorable for the glass to contain from 20 to 70%by weight, preferably from 30 to 60% by weight, of weakly basic oxides.

[0075] The cement according to the invention may if appropriate containstrongly basic oxides in an amount in the range from 0 to 25% by weight,preferably in the range from 0 to 10% by weight.

[0076] The cement according to the invention preferably has a flexuralstrength in the region of at least 25 MPa to 35 Mpa, particularlypreferably of greater than 45 MPa, measured in accordance with ISO 4049.

[0077] The working time of the cement, which is determined using aviscometer, is 1 to 4 min, particularly preferably 2 to 3 min. Thehardening time is 3 to 10 min, particularly preferably 4 to 8 min.

[0078] Preferred compositions of the glasses are given below.

[0079] In addition to the abovementioned weakly basic oxides from thescandium series, the glasses may also contain oxides from transitiongroups 4 and 5. Also, the aluminum oxide may be partially or completelyreplaced by boron oxide or gallium oxide. The melting conditions can bepositively influenced by the addition of oxides from the first maingroup, phosphate, and/or basic oxides from the second main group or ZnO.

[0080] The oxides which are separated from one another by “+” in thetable, may, according to the invention, also be present justindividually. The crucial factor is the corresponding proportion byweight which the group forms in the glass Oxide Proportion Y₂O₃ +La₂O₃ + other 30 to 80% by weight, preferably lanthanide oxides 35 to60% by weight B₂O₃ + Al₂O₃ + Ga₂O₃  5 to 50% by weight, preferably 10 to40% by weight, particularly preferably 15 to 35 SiO₂ + GeO₂ + SnO 10 to50% by weight, preferably 15 to 50% by weight P₂O₅  0 to 15% by weight,preferably  0 to 10% by weight, particularly preferably 0 to 2% byweight MgO + CaO + SrO + ZnO + BaO  0 to 10% by weight, preferably  0 to8% by weight, particularly preferably 0 to 5% by weight Li₂O + Na₂O +K₂O + Rb₂O + Cs₂O  0 to 5% by weight, preferably  0 to 3% by weight,particularly preferably 0 to 2% by weight TiO₂ + ZrO₂ + HfO₂  0 to 10%by weight, preferably  0 to 4% by weight, V₂O₅ + Nb₂O₅ + Ta₂O₅  0 to 10%by weight, preferably  0 to 4% by weight,

[0081] Instead of the “Y₂O₃+La₂O₃+other lanthanide oxides”, it is alsopossible for Sc₂O₃ to be present in an amount of from 20 to 50% byweight, preferably from 20 to 30% by weight. Glass compositions in whichSc₂O₃ is present in addition to the above constituent in a relativelysmall amount are also included.

[0082] The invention is explained in more detail below with reference toa number of examples.

[0083] None of the glasses described in the examples was treated with aninorganic acid leading to a reduction in the number of reactive ions atthe surface of the glasses (acid wash) before being reacted with apolyacid.

[0084] Production of the Glass:

[0085] Glasses of the following oxidic compositions (in % by weight)were melted at temperatures in the range from 1300 to 1600° C. over aperiod of 30 min to 5 h. With the exception of Comparative Example 4,the fluorine content in the starting batch was 12 to 14% by weight.

[0086] In Comparative Example 4, a glass was melted in accordance withDE 20 65 824 A1 using the following composition:

[0087] 9.5 g of SiO₂, 10.0 g of Al₂O₃, 7.6 g of Na₃AlF₆, 9.4 g of LaF₃,7.3 g of AlPO₄. The table gives the oxidic composition used in theseexamples. 1 2 3 4 5 6 7 8 9 C1 C2 C3 C4 C5 C6 C7 SiO₂ 20 19 20 17 32 2713 21 19 20 18 46 23 17 26 29 Sc₂O₃ Y₂O₃ 36 58 20 41 50 46 49 48 30La₂O₃ 48 63 20 34 22.5 23 CaO 7 16 47 3 16 SrO 17 Al₂O₃ 44 26 22 20 2832 37 28 26 29 34 35 36 26 30 27 P₂O₅ 6 10 5 ZrO2 5 Na₂O 1 1 1 2 8.5 6 1Li2O 13

[0088] Milling

[0089] 60 to 80 g of the glass granules obtained were dry-milled for 40to 50 min a vibratory agate mill (produced by Siebtechnik, millingvessel 100 ml, 910 rpm). The glass powders obtained had a mean grainsize in the range from 3 to 6 μm with a specific surface area of from1.8 to 2.5 m²/g.

EXAMPLE 6

[0090] Glass Example 6 was additionally wet-milled using a stirred ballmill. An aluminum oxide vessel (500 ml) was filled with 50 g of glasspowder, 200 ml H₂O and 100 g of zirconia balls (D=0.8 mm) and millingwas carried out for 6 h using a perforated zirconia disk. The result wasa mean grain size of 1.5 μm and a specific surface area of 10.5 m².

[0091] Cements

[0092] The cements were produced as a result of the glass powdersobtained being mixed with polyacids. In this step, an approximately 45%strength polyacrylic acid (molecular weight 20,000 to 30,000), anapproximately 45% strength polyacrylomaleic acid (molecular weightapprox. 40,000 to 60,000) and an approximately 55% strengthpolyvinylphosphonic acid (molecular weight approximately 20,000) wereused.

[0093] The setting was determined firstly in accordance with ISO 9917and secondly using the viscometer described in EP 0 023 013 A1. In allcases, the test assembly ensured that the temperature of the specimenswas controlled at 28° C. Flexural strengths were determined using thethree-point bending test on 2×2×25 mm cement specimens in accordancewith ISO 4049.

[0094] Results:

[0095] Cement 1:

[0096] Glass 1 was mixed both with a polyacrylic acid and apolyacrylomaleic acid with a P:F of 3:1. Polyacrylomaleic Polyacrylicacid acid Viscometer  3:30  3:50 (working time) Viscometer  9:10  9:00(hardening time) ISO 9117  8:30  7:30 Flexural strength 39.4 31.8 [Mpa]

[0097] Cement 6:

[0098] Glass 6 was reacted with polyacrylic acid (45% strength) with aP:F of 2.0:1. Stirred-ball Dry milling milling Viscometer  5:00  2:10(working time) Viscometer 12:00  7:45 (hardening time) ISO 9117 10:30 4:00 Flexural strength 27.9 MPa 41.5 MPa

[0099] Glass C6:

[0100] The glass, in one case untreated and in one case afterconditioning for 6 hours at 400° C. in a circulating air oven (producedby Heraeus), was reacted with polyacrylic acid. untreated conditionedViscometer not determinable  1:50 (working time) Viscometer  5:20(hardening time) ISO 9117 not determinable  5:00 Flexural strength notdeterminable 37.4 MPa

[0101] Further Cement Examples: Glass 2 3 4 5 7 8 9 C1 C2 C3 C4 C5 C7P:F 3:1  3:1  3:1  3:1  3.5:1   4:1  4:1  3:1   3:1  3:1  3:1  4:1  4:1Viscometer 1:30 2:50 2:40 3:20  4:00 3:50 2:50 0:45 <1:0 <1:0 <1:0 <1:0<1:0 (working time) Viscometer 3:50 8:20 5:20 7:40  9:10 8:50 6:45 1:30<1:0 <1:0 <1:0 <1:0 <1:0 (hardening time) ISO 9917 4:00 7:15 5:00  8:307:40 6:00 1:00 <1:0 <1:0 <1:0 <1:0 <1:0 Three- 37.4 41.6 34.8 35.6 32.943.9 — — — — — — point flexural strength

[0102] The setting times determined for the cements obtained in Examples1 to 9 are all within the preferred range, with the exception of Glass6, which was only dry-milled.

[0103] The cement in accordance with Comparative Example 1 sets tooquickly, presumably on account of the high Ca content. With the cementsaccording to Comparative Examples 2 and 3, measurement was no longerpossible, on account of setting taking place too quickly.

[0104] With the measurements using the viscometer, the first timecorresponds to the working time and the second time corresponds to thehardening time. The times given are in minutes. The P:F ratio is givenas a weight ratio.

[0105] The cements according to the invention are usually marketedpackaged in vessels. In this context, it should be ensured that theindividual components of the cement are in a form which is such thatthere is no undesirable reaction before they arrive at their intendeduse. The vessels usually have at least two chambers which are separatedfrom one another. Examples of suitable vessels are described in WO00/30953 A or EP 0 783 872 A.

[0106] Suitable vessels are mixing capsules and closeable box-likehollow bodies, such as screw-capped jars. Depending on the particularapplication, the cements may also be packaged in capsules.

1. A dental material comprising a dental glass characterized by thefollowing composition: Oxide Proportion Y₂O₃ + La₂O₃ + other lanthanideoxides 30 to 80% by weight B₂O₃ + Al₂O₃ + Ga₂O₃  5 to 50% by weightSiO₂ + GeO₂ + SnO 10 to 50% by weight P₂O₅  0 to 18% by weight MgO +CaO + SrO + ZnO + BaO + Li₂O +  0 to 10% by weight Na₂O + K₂O + Rb₂O +Cs₂O TiO₂ + ZrO₂ + HfO₂  0 to 10% by weight V₂O₅ + Nb₂O₅ + Ta₂O₅  0 to10% by weight

in which instead of the Y₂O₃+La₂O₃+other lanthanide oxides it is alsopossible for Sc₂O₃ to be present in an amount of from 20 to 50% byweight.
 2. The dental material as claimed in claim 1, comprising adental glass characterized by the following composition: OxideProportion Sc₂O₃ + Y₂O₃ + La₂O₃ + other lanthanide 30 to 60% by weightoxides Al₂O₃ 15 to 40% by weight SiO₂ 15 to 50% by weight P₂O₅  0 to 2MgO + CaO + SrO + ZnO + BaO  0 to 8% by weight Li₂O + Na₂O + K₂O +Rb₂O + Cs₂O  0 to 2% by weight TiO₂ + ZrO₂ + HfO₂  0 to 4% by weightV₂O₅ + Nb₂O₅ + Ta₂O₅  0 to 4% by weight

in which instead of the Y₂O₃+La₂O₃+other lanthanide oxides it is alsopossible for Sc₂O₃ to be present in an amount of 20 to 50%.
 3. Thedental material as claimed in one of the preceding claims, in which theglass is essentially a three-component system.
 4. The dental material asclaimed in one of the preceding claims, in which from 0 to 25% by weightof the oxygen in the dental glass is replaced by fluorine.
 5. The dentalmaterial as claimed in one of the preceding claims, in which the dentalglass is present in powder form with a specific BET surface area of 1 to15 m²/g.
 6. The dental material as claimed in one of the precedingclaims, in which the dental glass has a mean grain size in the rangefrom 0.01 to 10 μm.
 7. The dental material as claimed in one of thepreceding claims, in which the surface of the dental glass has not beensurface-treated, for example by washing with acid, surface coatingand/or conditioning, in order to adjust the setting time.
 8. A processfor producing a dental glass as described in one of the precedingclaims, comprising the steps a) providing the oxidic substances, b)mixing the oxidic substances, c) melting the mixture from step b), d)quenching the molten material to form a solid, e) milling the solid fromstep d) to form a glass powder, the glass powder from step d) not beingtreated with acid before it is used in a dental cement.
 9. The use of adental glass as described in one of the preceding claims for productionof a cement, without the surface of the glass having beingsurface-treated.
 10. The use as claimed in claim 9, in which the cementis a polyelectrolyte cement.
 11. A dental cement, comprising A) mineralsolid in an amount of from 50 to 90% by weight, B) water in an amount offrom 5 to 50% by weight, and C) polyacid in an amount of from 5 to 50%by weight, the mineral solid comprising a dental glass as described inone of claims 1 to
 8. 12. The dental cement as claimed in claim 11, inwhich in component A) the filler is present in an amount of from 0 to90% by weight.
 13. The dental cement as claimed in claim 12, in whichthe filler is selected from quartz, glasses, aluminum oxide, mineralpowders, such as feldspars or kaolin, and/or plastic powder.
 14. Thedental cement as claimed in one of claims 11 to 13 with a flexuralstrength of at least 25 Mpa, measured according to ISO
 4049. 15. Avessel having at least two chambers, containing a dental cement asclaimed in one of claims 12 to 14, in which the free-flowingconstituents are separated from the solid constituents.
 16. The vesselas claimed in claim 15 in the form of a mixing capsule.
 17. The use ofions of weakly basic-reacting oxides in glasses, which can enter into acement reaction with a polyacid, to crosslink the polyacid.
 18. The useas claimed in claim 17, in which the oxides are used in an amount of atleast 20% by weight.
 19. The use as claimed in one of claims 17 or 18,in which the oxides have a pK_(B) value of >1.
 20. The use as claimed inone of claims 17 to 19, in which the ions of the oxides are selectedfrom Sc³⁺, Y³⁺, La³⁺, Ce⁴⁺ and all the following trivalent andtetravalent lanthanides and Ga²⁺ and/or In²⁺.